The present invention relates to discharge lamps with an internal ionizable atmosphere containing mercury vapor, the vapor of at least one metal selected from group III A of the periodic table of elements, and at least one rare gas. It is characterized by the fact that this atmosphere also contains a halogenated organic compound, which combines with the selected metal when the tube is fully operating.
The invention relates more particularly to a lamp operating at medium pressure and rich in ultraviolet radiation. This lamp is used as a source of ultraviolet radiation for producing photo-chemical reactions, such as in particular the polymerization of polymerizable inks and varnishes used by printers or for coating the surfaces of objects of various materials such as wood, tissue, plastic or metallic materials, etc.
Among the various known advantages resulting from the processing of polymerizable inks and varnishes by ultraviolet radiation, mention may be made of the savings in power, the high processing speed and the reduction in pollution (noise and odors).
In general, earlier ultraviolet lamps were designed to meet the requirements of photographic or photosensitive papers. Compared to tests made with variable-spectrum lamps, it was found preferable for the polymerization of polymerizable inks and varnishes to provide a lamp whose radiated power in the 240 to 313 millimicrons ultraviolet is high with respect to that in the 334 to 408 millimicron ultraviolet range. The development of a lamp meeting these requirements constitutes a first objective of the invention.
In earlier techniques for printing drawings or copies, photographic or photosensitive paper was exposed by means of mercury-vapor lamps, whose maximum radiated energy was in the 365 to 407 millimicron range. In order to increase the intensity of the ultraviolet radiation, the spectral width had to be enlarged. This was achieved by adding metallic elements and a pure halogen, or again by means of metallic halides. A known method was to use, for example, metallic gallium and gaseous iodine or chlorine, or again a gallium halide in the form of gallium tri-iodide. A known disadvantage of these substances is the difficulty in adjusting the very small quantities required and of introducing them into the lamp. This is particularly true when chemically unstable compounds such as gallium tri-iodide are employed. Industrially, there is a danger of excessive statistical dispersion of the composition about the ideal or required values initially specified.
In another former type of UV lamp, the radiated spectrum was widened by associating metallic gallium and mercury iodide with the mercury. When this lamp was started, it was found that the mercury iodide decomposed into mercury and iodine, and that the iodine recombined with the gallium to produce gallium iodide. This method avoided the handling and measuring of gallium iodide or iodine.
One of the purposes of the invention is to overcome the above sources of error, by the introduction of a halide in the form of a stable compound easily measured.
Another purpose is to reduce losses in the lamp by reducing visible and infra-red radiation as far as possible, as well as far ultraviolet radiation which produces ozone.